In power supply and reference voltage design, Zener diodes have served as a "familiar companion" for nearly every engineer. However, their limitations have always been evident: their dynamic impedance often falls within the tens-of-ohms range; any fluctuation in the load or input voltage can cause the reference node voltage to swing; their temperature coefficient exhibits sharp variations with the breakdown voltage, particularly showing pronounced negative drift in the low-voltage region; more notably, their turn-on curve is relatively "soft," resulting in unstable voltage establishment at small currents and limited adjustment capability when used in parallel voltage regulation configurations. When systems impose stricter requirements on reference voltage accuracy, temperature drift, and load adjustment, this familiar companion begins to prove inadequate.

Corebai's proposed alternative solution is the CBM432—a three-terminal adjustable shunt regulator. Utilizing an active circuit design, it features a steep on-set curve, offering significantly superior voltage regulation stiffness compared to conventional low-voltage Zener diodes; its dynamic impedance is reduced to a typical value of 0.2 Ω, its reference voltage accuracy reaches 1.240 V ± 0.5%, and it can adjust the output voltage from 1.24 V up to 18 V using two external resistors. This device is ideally suited for applications within PC motherboards, voltage adapters, switching power supplies, and chargers where stable reference voltage or shunt regulation is required.

Architecture and Core Parameters

The CBM432 contains an internally integrated parallel adjustment circuit with a reference: the REF pin provides the reference voltage, the CATHODE serves as the adjustment negative terminal, and the ANODE is connected to ground; the voltage is clamped by absorbing current from the cathode. Under conditions of 25°C and IK = 10 mA, the reference input voltage VREF ranges from 1.234 V to 1.246 V (typical value: 1.240 V); across the entire operating temperature range, the reference deviation is typically only 10 mV and at most 25 mV, corresponding to an initial tolerance on the order of 0.5% and excellent temperature drift characteristics.

The following are the key electrical characteristics at 25°C (taken from the electrical characteristics table):

The device's current absorption capability covers approximately 80 µA to 100 mA (the electrical tester specifies a minimum operating current of 60–80 µA and a continuous cathode current absolute maximum of 100 mA), with an absolute maximum cathode voltage of 20 V, and it can operate across the full temperature range of −40°C to +125°C.

Key highlights of the project

The sharp transition to ultra-low dynamic impedance provides the fundamental rationale for replacing Zener diodes. The active output circuit enables the CBM432 to exhibit a typical dynamic output impedance of 0.2 Ω and a maximum value of 0.4 Ω within its rated operating current range—far lower than the tens-of-ohms level typically associated with conventional Zener diodes. This means that when the input voltage or load current fluctuates, the voltage at the reference node remains virtually constant, resulting in significantly improved "stability" of the parallel voltage regulator.

The reference voltage exhibits controlled accuracy and temperature drift: a typical reference voltage of 1.240 V with an initial tolerance of ±0.5%, and a maximum reference voltage deviation of 25 mV across the full temperature range; combined with the thermal stability guaranteed by the integrated components over this entire temperature range, this device is well-suited for use as a precision reference on printed circuit boards, eliminating the need for repeated negative temperature drift compensation in low-voltage segments—as would be required with Zener diodes.

The output voltage is adjustable; a single device can replace an entire string. Using only two external resistors, R1 and R2, the output voltage can be set from VREF up to 18 V, allowing for matching with different bus voltages without requiring component part number replacements. The minimum operating current is only 60–80 µA, resulting in low power consumption in standby and light-load scenarios.

The thermal performance of the package and its thermal resistance must be carefully evaluated. The junction-to-environment thermal resistance ($R_{\theta JA}$) for both the SOT-23 and SOT-89 packages is 371.7 °C/W. Since these devices operate under parallel current absorption conditions, the cathode power dissipation $P = V_{KA} \times I_K$ is directly converted into junction temperature; therefore, a sufficient heat dissipation margin must be calculated when operating under high current absorption conditions.

Typical implementation scenarios

Typical applications cover two main areas: parallel voltage regulation and precision reference circuits:

· Parallel voltage reference / high-current parallel regulator: utilizes an external transistor to expand the current range, pushing the reference output capability to the ampere level (the electrical example demonstrates a linear regulator application: with Vin = 10–20 V and Io = 1 A, the load regulation is 25 mV/0.5%).

· Voltage monitoring and comparison threshold: used for the threshold of a single-power-supply comparator with temperature compensation, or for power supply under-voltage/over-voltage monitoring.

· Constant current source and constant sink current: utilizes the reference pin to set a fixed voltage, with a external resistor used to determine the constant current.

· Series bypass voltage regulator / Three-terminal fixed voltage regulator output control: provides an adjustable reference or remote sampling for linear regulators.

· Switching power supply feedback and TRIAC/SCR clamping: provides feedback clamping or overvoltage protection for adapters, chargers, and switching power supplies.

· Small-signal front-end components such as linear ohmmeters and stylus amplifiers: utilize their low-noise reference and stable characteristics.

The measured test data for the linear regulator example in the datasheet can serve as a reference: for Vin = 10–20 V and Io = 1 A, the linear regulation voltage is 53 mV (1.1%), the load regulation voltage is 25 mV (0.5%), and the efficiency at Vin = 15 V and Io = 1 A is 82%.

Board-Level Design Key Points

The CBM432 is a parallel-connected device, and its design must ensure that it always carries a minimum operating current. When Vk = VREF, the typical minimum operating current is 60 µA and the maximum is 80 µA; therefore, the upper bias resistor must be calculated based on the "worst-case scenario" (lowest input voltage, highest required cathode current) to ensure that the cathode current never falls below IK(MIN) under any operating condition—otherwise, the reference voltage cannot be established, and the turn-on behavior will become soft.

The output voltage is determined by the voltage divider formed by R1 (connected from the cathode to the reference pin) and R2 (connected from the reference pin to ground): Vout = VREF × (1 + R1/R2). The reference input current IREF typically ranges from 0.25 µA to 0.5 µA; this current superimposes on the current through R1, introducing a small error. Therefore, high-precision voltage divider resistors should have relatively large values, yet these values must still be significantly greater than the influence of IREF; simultaneously, the R1/R2 ratio increases the overall dynamic impedance of the circuit (|ZKA'| = |ZKA| × (1 + R1/R2)), necessitating a trade-off between accuracy and stiffness.

Stability is the most error-prone aspect of parallel voltage regulators. The device must maintain stability across a range of operating currents and load capacitances; the stability boundary is determined jointly by the cathode current and the load capacitance (the stability boundary curve is obtained using a 150 Ω load resistor). If the downstream stage is connected to a high-capacitance load, the appropriate capacitor or series small-resistance compensation should be selected based on the boundary curve. For low-noise applications, it is recommended to use either small-capacitance or large-capacitance capacitors to provide noise filtering and main-pole attenuation, thereby preventing the third-stage reference system from being forced into "zero" due to instability.